Image synthesizing apparatus and method

ABSTRACT

In an image synthesizer ( 1 ), code stream analyzers ( 10, 11 ), code block extraction units ( 12, 13 ) and EBCOT decoders ( 14, 15 ) work together to decode encoded code streams (D 10 , D 11 ) encoded according to the MPEG-2000 Standard and generate quantization coefficients (D 16 , D 17 ) for each code block. In a cross-fading unit ( 16 ), multipliers ( 17, 18 ) multiply the quantization coefficients (D 16 , D 17 ) by coefficients (α(t), (1−α(t))) and an adder ( 19 ) adds together the results of multiplication to provide a cross-fading quantization coefficient (D 20 ). An EBCOT encoder ( 20 ), rate controller ( 21 ) and code stream generator ( 22 ) work together to encode the cross-fading quantization coefficient (D 20 ) to provide a final encoded code stream (D 23 ). Therefore, the image synthesizer ( 1 ) can combine two encoded code streams easily and effectively with a reduced use of a memory capacity.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to an image synthesizingapparatus and method, for combining two images encoded according to theJPEG-2000 Standard, for example, and more particularly to an imagesynthesizing apparatus and method suitable for use in the cross fading.

[0003] This application claims the priority of the Japanese PatentApplication No. 2003-120367 filed on Apr. 24, 2003, the entirety ofwhich is incorporated by reference herein.

[0004] 2. Description of the Related Art

[0005] Conventionally, the cross fading is well-known as an imageprocessing technique for representing a transition from one image as awhole to another, for example (cf. Japanese Published Unexamined PatentApplication Nos. 2000-78467 and -184278). The cross-fading technique isused in the computer graphics, special playback in a broadcastequipment, special playback in a camcorder, image processing in a gamemachine, etc.

[0006] Normally, the cross fading is implemented by linearlyinterpolating pixels included in two different images and takingspatially corresponding positions in the images, respectively, andcombining the two images together.

[0007] Recently, more and more researches have been done of thetechniques of dividing an image into a plurality of frequency bands by aso-called filter bank including a high-pass filter and low-pass filterin combination to encode each of the frequency bands. Of suchtechniques, the wavelet transform coding is considered as a newpromising technique which will take the place of DCT (discrete cosinetransform) because a high compression results in no considerable blockdistortion as in the DCT. For example, the JPEG-2000 Standardestablished as an international standard in January, 2001 has attained agreater improvement in efficiency of coding than the conventional JPEGby adopting a combination of the wavelet transform and a high-efficiencyentropy coding (bit modeling and arithmetic coding, both in units of abit plane).

[0008] Note here that to form an encoded code stream of a cross-fadedimage from an encoded code stream of each of two images with the use ofthe above-mentioned conventional technique, it is necessary to decodethe encoded code streams according to the JPEG-2000 Standard, combinethe two decoded images thus acquired by the linear interpolation togenerate a cross-faded image, and encode the cross-faded image accordingto the JPEG-2000 Standard.

[0009] However, such a technique requires a memory for storing the twodecoded images and also a memory for storing the cross-faded image. Inaddition, it needs both an image decoder and image encoder, which complywith the JPEG-2000 Standard.

OBJECT AND SUMMARY OF THE INVENTION

[0010] It is therefore an object of the present invention to overcomethe above-mentioned drawbacks of the related art by providing an imagesynthesizing apparatus and method, capable of combining two encoded codestreams easily and effectively with a reduced use of the memory space.

[0011] The above object can be attained by providing an imagesynthesizing apparatus that synthesizes an encoded code stream byfiltering first and second input images, generating code blocks eachhaving a predetermined size via division of a subband resulted from thefiltering, generating, per code block, a bit plane including bits from amost significant bit to a least significant bit, generating a codingpass by bit modeling of each bit plane, making input of first and secondencoded code streams generated by making arithmetic coding within thecoding pass, and combining the first and second encoded code streams togenerate the synthetic encoded code stream, the apparatus including,according to the present invention, first and second image decodingmeans each including a code stream analyzing means for analyzing thefirst and second encoded code streams, a code block extracting means forextracting code block information on the basis of the result of analysisfrom the code stream analyzing means, and an arithmetic decoding meansfor making arithmetic decoding of the code block information; asynthesizing means for multiplying a coefficient value for each of thecode blocks supplied from the first and second image decoding means byfirst and second real-number values, respectively, and adding theresults of multiplication together; and an arithmetic coding means formaking arithmetic coding of the result of addition from the synthesizingmeans to generate the synthetic encoded code stream.

[0012] Also, the above object can be attained by providing an imagesynthesizing method in which an encoded code stream is synthesized byfiltering first and second input images, generating code blocks eachhaving a predetermined size via division of a sub band resulted from thefiltering, generating, per code block, a bit plane including bits from amost significant bit to a least significant bit, generating a codingpass by bit modeling of each bit plane, making input of first and secondencoded code streams generated by making arithmetic coding within thecoding pass, and combining the first and second encoded code streams togenerate the synthetic encoded code stream, the method including,according to the present invention, first and second image decodingsteps each including the steps of analyzing the first and second encodedcode streams, extracting code block information on the basis of theresult of analysis from the code stream analyzing means; and makingarithmetic decoding of the code block information; a synthesizing stepof multiplying a coefficient value for each of the code blocks suppliedfrom the first and second image decoding means by first and secondreal-number values, respectively, and adding the results ofmultiplication together; and an arithmetic coding step of makingarithmetic coding of the result of addition from the synthesizing meansto generate the synthetic encoded code stream.

[0013] In the above image synthesizing apparatus and method, two codestreams encoded according to the MPEG-2000 Standard for example, arecombined together to generate the synthetic encoded code stream, whichsynthesis being effected in a coefficient domain, not in any spatialdomain. Thus, the present invention permits to provide the same resultas that of the synthesis in a spatial domain only by utilizing a part ofan image decoder and encoder, that comply with the MPEG-2000 Standard,and with a smaller sharing of the memory capacity than in the synthesisin the spatial domain.

[0014] These objects and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments of the present invention whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 explains the concept of a conventional cross-fadingtechnique;

[0016]FIG. 2 is a schematic block diagram of a conventional imagesynthesizer in which the conventional cross-fading technique shown inFIG. 1 is adopted;

[0017]FIG. 3 explains subbands in wavelet transform down to a secondlevel;

[0018]FIG. 4 explains the relation between code blocks and subbands;

[0019]FIG. 5 explains a bit plane, in which FIG. 5A shows a quantizationcoefficient consisting of 16 coefficients in total, FIG. 5B shows a bitplane of the absolute values of the coefficient, and FIG. 5C shows a bitplane of codes;

[0020]FIG. 6 explains a procedure of processing a coding pass in thecode block;

[0021]FIG. 7 explains a procedure of scanning the coefficients in thecode block;

[0022]FIG. 8 is a schematic block diagram of an image synthesizer as anembodiment of the present invention;

[0023]FIG. 9 shows an example of a cross-faded image when α=0.2;

[0024]FIG. 10 shows an example of a cross-faded image when α=0.5; and

[0025]FIG. 11 shows an example of a cross-faded image when α=0.8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The present invention will be described in detail belowconcerning an embodiment thereof with reference to the accompanyingdrawings. Prior to starting the description of the present invention,however, there will be described a conventional technique of generatinga cross-faded image by combining two images and a conventional imagesynthesizer in which the conventional technique is applied for synthesisof images encoded according to the MPEG-2000 Standard.

[0027] Conventionally, a cross-faded image G(x, y, t) is generated froman image F₁(x, y, t) and image F₂(x, y, t) via linear interpolation ofsamples existent in identical positions in different frames at the sametime. The cross-faded image G(x, y, t) is represented as given by thefollowing formula (1):

G(x, y, t)=α(t)×F ₁(x, y, t)+(1−α(t))×F ₂(x, y, t)   (1)

[0028] where x and y indicate horizontal and vertical coordinates of animage and t indicates the time.

[0029] For application of the conventional technique for synthesis ofimages encoded according to the MPEG-2000 Standard, there is used animage synthesizer, generally indicated with a reference 100 in FIG. 2for example. As shown, the image synthesizer 100 is supplied with codestreams D100 and D101 encoded according to the MPEG-2000 Standard, andmakes cross fading of the code streams D100 and D101 to provide anencoded code stream D115, having thus undergone the cross fading.

[0030] In the image synthesizer 100, an EBCOT (embedded coding withoptimized truncation) decoder 101 decodes the encoded code stream D100to generate a quantization coefficient D102, and supplies it to adequantizer 103. This dequantizer 103 dequantizes the quantizationcoefficient D102 to generate a wavelet transform coefficient D104, andsupplies it to a wavelet inverse-transformer 105. The waveletinverse-transformer 105 makes wavelet inverse-transform of the wavelettransform coefficient D104 to generate a decoded image D106, andsupplies it to and a cross-fading unit 107.

[0031] Similarly, an EBCTO decoder 102 decodes the encoded code streamD101 to generate a quantization coefficient D103, and supplies it to adequantizer 104. The dequantizer 104 dequantizes the quantizationcoefficient D103 to generate a wavelength transform coefficient D105,and supplies it to a wavelet inverse-transformer 106. The waveletinverse-transformer 106 makes wavelet inverse-transform of the wavelettransform coefficient D105 to generate a decoded image D107, andsupplies it to the cross-fading unit 107.

[0032] The cross-fading unit 107 includes multipliers 108 and 109 and anadder 110. Making a calculation as given by the formula (1), thecross-fading unit 107 generates a cross-faded image D110. The multiplier108 multiplies the decoded image D106 by a coefficient α(t), while themultiplier 109 multiplies the decoded image D107 by a coefficient(1−α(t)). Then, the adder 110 is supplied with images D108 and D109 fromthe multipliers 108 and 109, respectively, adds them together to providea cross-faded image D110, and supplies the cross-faded image D110 to awavelet transformer 111. It should be noted that the decoded images D106and D107 and the cross-faded image D110 correspond to F₁(x, y, t), F₂(x,y, t) and G(x, y, t), respectively, in the above formula (1).

[0033] With the above operations, the cross-faded image D110 isgenerated from the input encoded code streams D100 and D101. In a systemdownstream of the system down to the wavelet inverse-transformer 111,the cross-faded image D110 is encoded according to the MPEG-2000Standard to generate an encoded code stream D115.

[0034] The wavelet transformer 111 is normally a filter bank including alow-pass filter and a high-pass filter. It should be noted that adigital filter has to be pre-buffered with a sufficient amount of inputimages for filtering since it normally shows an impulse response (filterfactor) for a plurality of tap lengths. However, no digital filter isillustrated in FIG. 2 because its configuration is simple.

[0035] The wavelet transformer 111 is supplied with a minimum necessaryamount of cross-faded images D110 for filtering and filters it forwavelet transform to generate a wavelet transform coefficient D111.

[0036] In the above wavelet transformation, a low-frequency component isnormally repeatedly transformed as shown in FIG. 3 because majority ofthe image energy is concentrated to the low-frequency component. Itshould be noted that the level number of the wavelet transform in FIG. 3is 2 (two), and thus a total of seven subbands is generated. Morespecifically, the horizontal size X_SIZE and vertical size Y_SIZE arehalved by a first filtering to provide four subbands LL1, LH2, HL2 andHH2. The subband LL1 is quartered by a second filtering to provide foursubbands LL0, LH1, HL1 and HH1. It should be noted that in FIG. 3, “L”and “H” indicate a low-frequency band and high-frequency band,respectively, and numbers suffixed to “L” and “H”, respectively,indicate resolution levels, respectively. That is, “LH1”, for example,indicates a subband having a resolution level of 1 (one) in which alow-frequency band extends horizontally while a high-frequency bandextends vertically.

[0037] The synthesizer 100 further includes a quantizer 112 that makesirreversible compression of the wavelet transform coefficient D111supplied from the wavelet transformer 111. This quantizer 112 may adopta scalar quantization to divide the wavelet transform coefficient D111by a quantization step size.

[0038] Also, the synthesizer 100 includes an EBCOT encoder 113 thatmakes an entropy coding, defined in the JPEG-2000 Standard and called“EBCOT”, of the quantization coefficient D112 for each of the subbandsgenerated by the quantizer 112 to generate an arithmetic code D113. TheEBCOT encoder 113 encodes the quantization coefficient D112 for each ofthe aforementioned code blocks. It should be noted that the EBCOT(embedded coding with optimized truncation) is described in detail in“ISO/IEC FDIS 15444-1, JPEG-2000 Part-1 FDIS, 18 Aug., 2000” and thelike.

[0039] More particularly, the EBCOT encoder 113 first divides thequantization coefficient D112 for each of the subbands generated by thequantizer 112 into code blocks that are units of coding defined in theJPEG-2000 Standard. Namely, code blocks each having a size of about64×64 are generated in each of the subbands after thus divided as shownin FIG. 4. It should be noted that the JPEG-2000 Standard defines thatthe size of a code block is expressed by a power of 2 both horizontallyand vertically and that a size of 32×32 or 64×64 is normally used inmany cases.

[0040] Then, the EBCOT encoder 113 makes, for each bit plane,coefficient bit modeling of the quantization coefficient for each codeblock as will be described below. The concept of this bit plane will bedescribed below with reference to FIG. 5. FIG. 5A shows an assumedquantization coefficient including a total of 16 coefficients (=4vertical coefficients by 4 horizontal coefficients). The largestabsolute-value one of these 16 quantization coefficients is 13(thirteen) that is binary-notated as “1101”. Therefore, the bit planesdefined by the coefficient absolute-values include four as shown in FIG.5B. It should be noted that all elements in each bit plane take a number0 (zero) or 1 (one). On the other hand, the only one of the quantizationcoefficients which has a negative sign is “−6”, while all the otherquantization coefficients are 0 (zero) and positive-signed ones.Therefore, the bit plane of signs is as shown in FIG. 5C.

[0041] Each of the code blocks is encoded per bit plane independently ina direction from the most significant bit (MSB) to least significant bit(LSB). A quantization coefficient is expressed by a signed binary numberof n bits, and bit 0 to bit (n-2) represent the bits, respectively,included between LSB and MSB. It should be noted that the remaining onebit is a sign. The code blocks are sequentially encoded starting withthe MSB-side bit plane via three types of coding passes as shown below:

[0042] (a) Significant propagation pass (also called SP pass)

[0043] (b) Magnitude refinement pass (also called MR pass)

[0044] (c) Clean-up pass (also called CU pass)

[0045] The three types of coding passes are used in a sequence as shownin FIG. 6. As shown in FIG. 6, a bit plane (n-2) at the MSB side isfirst encoded via the CU pass. Next, bit planes are sequentially encodedtoward the LSB side. The bit planes are encoded via the SP pass, MR passand CU pass in this order.

[0046] Actually, however, it is written in a header in which bit planecounted from the MSB there will appear “1”, and all-zero bit planes willnot be encoded. The three types of coding passes are repeatedly used inthis order to encode the bit planes, and the encoding is ceased after anarbitrary bit plane is encoded via an arbitrary one of the codingpasses. Thereby, a tradeoff can be made between the bit rate and imagequality, namely, the bit rate can be controlled.

[0047] The coefficients are scanned as will be described below withreference to FIG. 7. The code blocks are grouped at each height of fourcoefficients into a stripe. The stripe is as wide as the width of thecode block. The “scanning sequence” means a sequence in which allcoefficients in one code block are scanned. In a code block, thecoefficients are scanned in a sequence from the upper to lower stripe.In each stripe, the coefficients are scanned in a sequence from the leftto right row. In each of the rows, the coefficients are scanned in asequence from the top to bottom. It should be noted that in each codingpass, all the coefficients in a code block are scanned in thesesequences of scanning.

[0048] As above, the EBCOT encoder 113 decomposes the quantizationcoefficient in each code block into bit planes, each of the bit planesinto three coding passes, and generates a quantization coefficient foreach of the coding passes. Then, the EBCOT encoder 113 makes arithmeticcoding of the quantization coefficient for each coding pass.

[0049] The image synthesizer 100 further includes a rate controller 114that controls the bit rate to approximate a target bit rate orcompression ratio while counting the amount of the arithmetic codes D113supplied from the EBCOT encoder 113. More specifically, the ratecontroller 114 controls the bit rate by truncating at least a part ofthe coding pass for each code block.

[0050] The image synthesizer 100 also includes a code stream generator115 that packetizes the rate-controlled arithmetic code D 114 suppliedfrom the rate controller 114 according to the JPEG-2000 Standard, andadds a header to the packet to provide a final encoded code stream D115.

[0051] As above, in the image synthesizer 100, the two encoded codestreams encoded according to the MPEG-2000 Standard, are supplied forcross fading. When outputting the encoded code streams after crossfading, two images are combined in a spatial domain to generate across-faded image, then the cross-faded image is encoded to generate across-faded encoded code stream.

[0052] For the image synthesizer 100 configured as above, however, thereshould be used a memory to store the two decoded images and also amemory to store the cross-faded image. Also, the image synthesizer 100needs an image decoder and image encoder, both complying with theJPEG-2000 Standard.

[0053] The image synthesizer according an embodiment of the presentinvention makes cross fading in the coefficient domain, not in thespatial domain, to overcome the above-mentioned drawbacks of theconventional image synthesizer. This will be explained herebelow.

[0054] Referring now to FIG. 8, there is schematically illustrated inthe form of a block diagram the image synthesizer as the embodiment ofthe present invention. As shown in FIG. 8, the image synthesizer as theembodiment of the present invention is generally indicated with areference 1. As shown, it includes code stream analyzers 10 and 11, codeblock extraction units 12 and 13, EBCOT decoders 14 and 15, cross-fadingunit 16, EBCOT encoder 20, rate controller 21, and a code streamgenerator 22. The cross-fading unit 16 includes multipliers 17 and 18and an adder 19.

[0055] The code stream analyzer 10 is supplied with a code stream D10,encoded according to the JPEG-2000 Standard, and analyzes the encodedcode stream D10 with a technique defined in the MPEG-2000 Standard. Thecode block extraction unit 12 supplies encoded information D14 for eachcode block to the EBCOT decoder 14 according to analysis information D12supplied from the code stream analyzer 10. The EBCOT decoder 14 decodesthe encoded information D14 to generate a quantization coefficient D16for each code block, and supplies the quantization coefficient D16 tothe cross-fading unit 16.

[0056] Similarly, the code stream analyzer 11 is supplied with a codestream D11, encoded according to the JPEG-2000 Standard, and analyzesthe encoded code stream D11 with a technique defined in the MPEG-2000Standard. The code block extraction unit 13 supplies encoded informationD15 for each code block to the EBCOT decoder 15 according to analysisinformation D13 supplied from the code stream analyzer 11. The EBCOTdecoder 15 decodes the encoded information D15 to generate aquantization coefficient D17 for each code block, and supplies thequantization coefficient D17 to the cross-fading unit 16.

[0057] The cross-fading unit 16 includes the multipliers 17 and 18 andadder 19. Combining the quantization coefficients D16 an dD17, thecross-fading unit 16 generates a cross-fading quantization coefficientD20. More specifically, on the assumption that the quantizationcoefficient D16 is Q_cb1(x, y) and quantization coefficient D17 isQ_cb2(x, y), the cross-fading unit 16 generates a cross-fadingquantization coefficient using the following formula (2).

[0058] It should be noted that since Q_cb1(x, y) and Q_cb2(x, y) areassumed to be at the same time, no time t is necessary as a parameter asin the above formula (1):

G _(—) Q(x, y)=α(t)×Q _(—) cb1(x, y)+(1−α(t))×Q _(—) cb2(x, y)   (2)

[0059] where x and y indicate horizontal and vertical positions,respectively, of the quantization coefficient domain.

[0060] That is, the multiplier 17 multiplies the quantizationcoefficient D16 by a coefficient α(t), and multiplier 18 multiplies thequantization coefficient D17 by a coefficient (1−α(t)). The adder 19adds the quantization coefficients D18 and D19 supplied from themultipliers 17 and 18 to provide a cross-fading quantization coefficientD20, and supplies the cross-fading quantization coefficient D20 to theEBCOT encoder 20.

[0061] The EBCOT encoder 20 makes EBCOT entropy coding of thecross-fading quantization coefficient D20 from the cross-fading unit 16to generate an arithmetic code D21.

[0062] The rate controller 21 controls the bit rate to approximate atarget bit rate or compression ratio while counting the amount of thearithmetic codes D21 supplied from the EBCOT encoder 20. Morespecifically, the rate controller 21 controls the bit rate by truncatingat least a part of the coding pass for each code block. It should benoted that the arithmetic code D21 may be supplied as it is to the codestream generator 22 while controlling the bit rate. In this case, theimage synthesizer 1 does not need the rate controller 21.

[0063] The code stream generator 22 packetizes the rate-controlledarithmetic code D22 supplied from the rate controller 21 according tothe JPEG-2000 Standard, and adds a header to the packet to provide afinal encoded code stream D23.

[0064] The encoded code streams D10 and D11 are ones resulted fromcoding of an parrot image and a house-including landscape. FIGS. 9 to 11show cross-faded images processed by the cross-fading unit 16 withα=0.2, α=0.5 and α=0.8, respectively. As seen in FIGS. 9 to 11, thehouse-including landscape and parrot image appear smoothly faded. Itshould be noted that FIGS. 9 to 11 show images resulted from crossfading with three values of α: α=0.2, α=0.5 and α=0.8 but the smoothnessof cross fading can be changed in degree by changing the ratio in timechange among the values of α(t).

[0065] As having been described in the foregoing, the image synthesizer1 as the embodiment of the present invention makes cross fading of inputtwo code streams encoded according to the JPEG-2000 Standard to providea cross-faded encoded code stream. The cross fading in the coefficientdomain can provide the same result as that of a cross fading in thespatial domain, and uses only a part of the image decoder and encoderthat comply with the JPEG-2000 Standard.

[0066] Also, the cross fading in the coefficient domain advantageouslyuses the memory capacity less than the cross fading in the spatialdomain. In particular, since the image synthesizer 1 as the embodimentof the present invention makes cross fading for each code block, so itcan make the cross fading with a rather smaller use of the memorycapacity than that in the cross fading made for an entire image.

[0067] In the foregoing, the present invention has been described indetail concerning certain preferred embodiments thereof as examples withreference to the accompanying drawings. However, it should be understoodby those ordinarily skilled in the art that the present invention is notlimited to the embodiments but can be modified in various manners,constructed alternatively or embodied in various other forms withoutdeparting from the scope and spirit thereof as set forth and defined inthe appended claims.

[0068] For example, in the aforementioned image synthesizer 1, the imagedecoding means (code stream analyzer 10, code block extraction unit 12and EBCOT decoder 14) provided for decoding the encoded code stream D10down to the quantization coefficient D16, and the image decoding means(code stream analyzer 11, code block extraction unit 13 and EBCOTdecoder 15) provided for decoding the encoded code stream D11 down tothe quantization coefficient D17, may be separately provided or may beincluded in one image decoder. In the latter case, the image decodingcan be parallelized using the technique called “pipeline processing”used in many hardware.

What is claimed is:
 1. An image synthesizing apparatus that synthesizesan encoded code stream by filtering first and second input images,generating code blocks each having a predetermined size via division ofa subband resulted from the filtering, generating, per code block, a bitplane including bits from a most significant bit to a least significantbit, generating a coding pass by bit modeling of each bit plane, makinginput of first and second encoded code streams generated by makingarithmetic coding within the coding pass, and combining the first andsecond encoded code streams to generate the synthetic encoded codestream, the apparatus including according to the present invention:first and second image decoding means each including: a code streamanalyzing means for analyzing the first and second encoded code streams;a code block extracting means for extracting code block information onthe basis of the result of analysis from the code stream analyzingmeans; and an arithmetic decoding means for making arithmetic decodingof the code block information; a synthesizing means for multiplying acoefficient value for each of the code blocks supplied from the firstand second image decoding means by first and second real-number values,respectively, and adding the results of multiplication together; and anarithmetic coding means for making arithmetic coding of the result ofaddition from the synthesizing means to generate the synthetic encodedcode stream.
 2. The apparatus according to claim 1, wherein the sum ofthe first and second real-number values is 1 (one).
 3. The apparatusaccording to claim 1, wherein the synthesizing means adds ones, of thecoefficient values for the code blocks, multiplied by the first andsecond real-number values, respectively, that are existent incorresponding positions.
 4. The apparatus according to claim 1, whereinboth the first and second image decoding means form the same imagedecoding means.
 5. The apparatus according to claim 1, wherein the firstand second real-number values vary at variable rates, respectively. 6.An image synthesizing method in which an encoded code stream issynthesized by filtering first and second input images, generating codeblocks each having a predetermined size via division of a sub bandresulted from the filtering, generating, per code block, a bit planeincluding bits from a most significant bit to a least significant bit,generating a coding pass by bit modeling of each bit plane, making inputof first and second encoded code streams generated by making arithmeticcoding within the coding pass, and combining the first and secondencoded code streams to provide the synthetic encoded code stream, themethod comprising: first and second image decoding steps each includingthe steps of: analyzing the first and second encoded code streams;extracting code block information on the basis of the result of analysisfrom the code stream analyzing means; and making arithmetic decoding ofthe code block information; a synthesizing step of multiplying acoefficient value for each of the code blocks supplied from the firstand second image decoding means by first and second real-number values,respectively, and adding the results of multiplication together; and anarithmetic coding step of making arithmetic coding of the result ofaddition from the synthesizing means to generate the synthetic encodedcode stream.